The technical field of the present invention relates to packaging materials, and more specifically to multilayer packaging materials having at least one layer of a gas-permeable substrate and at least one layer of a gas barrier coating.
Plastics have found increasing use as replacements for glass and metal containers in packaging. Advantages of such plastic packaging over glass packaging include lighter weight, decreased breakage and potentially lower costs. Moreover, an advantage of plastic packaging over metal packaging is that the former can more easily be designed as re-closable. Notwithstanding the above, shortcomings in the gas barrier properties of common plastic packaging materials (e.g., polyesters, polyolefins and polycarbonates) present major problems to those in the packaging industry when such materials are used to package oxygen-sensitive items and/or carbonated beverages.
Specifically, gases such as oxygen and carbon dioxide can readily permeate through most of the plastic materials commonly used by the packaging industry. The oxygen permeability constant (herein referred to as "OPC") of such plastic materials quantifies the amount of oxygen which can pass through a film or coating under a specific set of circumstances and is generally expressed in units of cubic centimeter-mil/100 square inches/atmosphere/day. This is a standard unit of permeation measured as cubic centimeters of oxygen permeating through 1 mil (25.4 micron) thickness of a sample, 100 square inches (645 square centimeters) in area, over a 24-hour period, under a partial pressure differential of one atmosphere at specific temperature and relative humidity (R.H.) conditions. As used herein, OPC values are reported at 30.degree. C. and 50% R.H. unless otherwise stated.
Since many foods, beverages, chemicals, medicines, medical supplies and the like are sensitive to oxidation, they typically must be protected from the ingress of oxygen into the container in which they are stored so as to prevent their discoloration and/or spoilage. Moreover, carbonated beverages should also be stored in sealed containers which prevent the egress of carbon dioxide therefrom so as to prevent the beverage from going flat. As used herein, the term "flat" refers to a carbonated beverage losing at least about 10% of its carbonation, typically at least about 15% of its carbonation, and more typically at least about 20% of its carbonation. Accordingly, since oxygen and carbon dioxide can readily permeate through most plastic materials used by the packaging industry, the shelf-life of items stored in conventional plastic containers is reduced when compared to their shelf-life when stored in glass or metal containers.
Some examples of oxygen sensitive items whose shelf-life would be greatly reduced if stored in conventional plastic containers are perishable foods and beverages such as tomato-based products (e.g., ketchup, tomato sauces and tomato pastes), juices (e.g., fruit and vegetable juices) and carbonated alcoholic beverages (e.g., beer, ale, malt beverages, sparkling wines, champagnes, and the like). In these instances, exposure to minute amounts of oxygen over a relatively short period of time can adversely affect their taste. Some examples of carbonated beverages whose shelf-life would be greatly reduced if stored in conventional plastic containers are soft drinks, malt beverages, sparkling water, sparkling wines, champagne, and the like.
One of the common packing materials used today by the food and beverage industry is poly(ethylene teraphthalate) ("PET"). Notwithstanding its widespread use, PET has a relatively high OPC value (i.e., about 6.0). As such, the food and beverage packaging industry has sought ways to improve the OPC value of such packaging materials. It should be noted that, typically, oxygen permeates through a film and/or coating more readily than does carbon dioxide. Accordingly, although OPC values pertain to the permeability of oxygen through a film and/or coating, lowering a coating's OPC value improves not only its oxygen barrier properties, but also its carbon dioxide barrier properties.
One of the methods disclosed in the literature as a means of improving a plastic packaging material's OPC value pertains to chemically and/or physically modifying the plastic. This method is typically expensive and can create recycleability problems. Another method disclosed in the literature as a means of improving a plastic packaging material's OPC value pertains to coating the plastic material with a gas barrier material (e.g., a gas barrier coating composition or a gas-barrier film). This method is typically less expensive than that set out above and creates fewer, if any, recycleability problems.
Numerous gas barrier coating compositions have been disclosed in the prior art. For example, polyepoxide-polyamine based gas barrier coating compositions having very low OPC values are the subject of commonly-owned U.S. Pat. Nos. 5,006,381; 5,008,137 and 5,300,541 and WO 95/26997. These coatings have found commercial acceptance as barrier coatings for application over conventional polymeric containers. However, further improvements are still desirable by certain segments in the packaging industry. An example of such an improvement would include the development of gas barrier coatings that have OPC values of less than 0.06 and a smooth and glossy appearance.
For example, the malt beverage industry has established very strict quality standards for small beverage containers (e.g., 12 ounce (355 milliliter) bottles made out of PET having an average wall thickness of 15 mils (381 microns)). According to this shelf-life standard, typically not more than 5 ppm of oxygen should pass through the walls of the sealed container over a 90-day storage period at ambient temperatures and 50% R.H. Parts per million of oxygen is based upon the weight of oxygen to the weight of the beverage (1 cubic centimeter of oxygen weighs 0.0014 gram). For example, one cubic centimeter of oxygen in 12 ounces of beverage would be 4.0 ppm ((0.0014 grams per cubic centimeter of oxygen/355 cubic centimeters in a 12 ounce bottle).times.10.sup.6). Preferred levels of performance for the malt beverage industry would entail that, over the 90-day storage period at ambient temperatures and 50% R.H., not more than 4 ppm oxygen, more preferably not more than 3 ppm oxygen, and even more preferably not more than 2 ppm of oxygen pass through the walls of the sealed container.
One way in which a polymeric packaging material comprising PET can meet the aforementioned malt beverage industry shelf-life standard of allowing not more than 5 ppm of oxygen from passing through its walls over a 90-day period when stored at ambient temperatures and 50% R.H., is to coat the packaging material with a gas barrier coating which has an OPC value of not more than 0.05. Moreover, a way in which a polymeric packaging material comprising PET can meet the preferred malt beverage industry standard of allowing not more than 4 ppm of oxygen, more preferably not more than 3 ppm of oxygen, and the even more preferably not more than 2 ppm of oxygen from passing through the gas barrier coating over a 90-day period when stored at ambient temperatures and 50% R.H., is to coat the packaging material with a gas barrier coating which has an OPC value of not more than 0.04, more preferably of not more than 0.03, and even more preferably of not more than 0.02, respectively. Notwithstanding the advantages associated with using polymeric materials for making malt beverage containers, for reasons such as high cost, insufficient OPC values, and/or poor appearance of conventional gas barrier coatings, the malt beverage industry continues to make malt beverage containers out of glass and/or metal.
It is known that malt beverages are not stable in light with wavelengths of electromagnetic radiation ranging from 300 nanometers (nm) to 500 nm (hereinafter referred to as "product damaging light"). It is also known that brown or dark amber-tinted glass substantially blocks most of this product damaging light. As used herein, the term "substantially blocks" means that less than about 10%, preferably less than about 7%, more preferably less than about 5% and even more preferably less than about 3% of this product damaging light passes there through.